LibCrypto: Add SHA512

There is quite a bit of avoidable duplication, however, I could not get
the compiler to be happy about SHA2<Size> (see FIXMEs)

Libraries/LibCrypto/Hash/HashFunction.h

@@ -48,7 +48,9 @@ namespace Hash {
         virtual void update(const ByteBuffer& buffer) = 0;
         virtual void update(const StringView& string) = 0;
 
+        virtual DigestType peek() = 0;
         virtual DigestType digest() = 0;
+
         virtual String class_name() const = 0;
     };
 }